Just do a video call with the employee when they request e.g. MFA resets. Have a policy that enforces that. Maybe have them show an ID.
- compare the person with an ID on file
- invite the manager or a colleague of the person into the call to confirm
- ask for PII (birthdays, address) that are on file
I assume someone assuming the identity is more reluctant to show themselves on video. Furthermore being asked PII they are likely not as prepared.